What’s this black stuff???

[Xxaprilrose]

I’ve had my flowerhorn for about three weeks. He’s only about 6 inches so he’s still very young. Yesterday I noticed on his face (and only his face) he has some black marks??? Is this just coming in as part of his pattern or is something wrong? All my water parameters are good. Ph 7.6 temp 84 ammonia 0 nitrite 0 nitrate 10. However two days ago something happened idk what but I had a nitrite spike for not even 24 hours. It spiked up to .50ppm and I dosed it quickly once I saw the spike with prime and the next day no nitrites. I also added more biomedia to the filter that I had been meaning to do so that may have helped a tad. Before prime I didn’t notice any black marks it wasn’t until the next day when all parameters were normal. However could it be burns from nitrites?? Or something else? He does try to jump at the top which is black but the top isn’t painted so I don’t think it would be any type of paint on him… idk any help thank u!